Crews responded to reports of thick smoke and large flames at a building near Larkin and California streets around 5:00 p.m.
The building is an apartment complex with nine units.
All nine units are deemed uninhabitable, and Red Cross is on scene assisting those who were displaced by the fire.
Everyone made it out safely, except for one cat who did not survive.
San Francisco Fire Department is working to determine what started the fire, but a neighbor tells ABC7 News that it smelled like someone was barbecuing inside.
Another witness, who was walking past the outside of the building, says she saw dark, thick smoke and then a sudden outburst of huge flames.
Cable cars are not able to pass through the area at this time.
Stay with ABC7 News for updates.